1. Field of the Invention
The present invention relates to an image forming apparatus, a normal image forming unit, a special image forming unit and an image forming method in which an image is formed on a recording medium such as paper. More specifically, it relates to an image forming apparatus, a normal image forming unit, a special image forming unit and an image forming method in which reproduction preventing information can be printed.
2. Description of the Related Art
Various image forming apparatus have currently been used. Typical examples include an inkjet printer and a laser printer.
In the inkjet printer, an image is formed by directly jetting ink droplets to a printing medium. Further, in the inkjet printer, setup of printing with a low or medium precision in the case of printing on plain paper and setup of printing with a relatively high precision in the case of printing on special paper such as coated paper are adapted to be switchable.
Meanwhile, in the laser printer, a charged photosensitive drum is exposed to form an electrostatic latent image thereon, a toner image is formed on the electrostatic latent image by a developing device, and the toner image formed on the photosensitive drum is transferred onto a printing medium to form the image on the printing medium.
To such an image forming apparatus, it is required to impart a function of preventing leak of information caused by reproduction. For example, there is a need to provide reproduction preventing information for preventing reproduction of a document including confidential information on original documents or certified copies of original documents.
As an example of a method of attaining such a function of preventing leak of information, there is a method using latent images and (or) characters. In this method, image formation of a reproduction preventing information is conducted at the same time confidential information is printed on copy-proof paper. In this case, reproduction preventing information is formed using latent images and (or) characters to be concealed on the copy-proof paper. When it is reproduced, the latent image appears on the copy. Consequently, it is obviously found that confidential information is reproduced on the paper.
The method of preventing leak of information using the copy-proof paper is effective for inhibiting reproduction. However, since the reproduction preventing information is formed on an original document or its certified copy (copy-proof paper) as a latent image, it cannot visually be observed (lacking in obviousness). Accordingly, it is impossible to confirm at once whether a document is a true original document (whether it is not a document having been forged or tampered with) or which original document is requested when there are plural original documents.
In this instance, a method in which reproduction preventing information describing a history of information itself to be protected, incapable of reproduction and having obviousness, namely, reproduction preventing information is formed as an image having a high precision on a printing medium is considered to increase an effect of preventing leak of information and to prevent forgery or tampering. However, for forming the high-precision image, it is necessary to employ a printing method using an artwork or the like. In the case of using an artwork, each piece of image information requires the corresponding artwork, and it is impossible to print reproduction preventing information corresponding to each printing medium on demand. Further, in view of a precision of an apparatus, it is impossible to form high-precision security information incapable of reproduction in the printing method using the artwork.
Meanwhile, for forming a high-precision image incapable of reproduction in an inkjet printer, there is a need to further mninimize ink droplets, but it gives rise to a problem that increase of an amount of image information notably decreases a printing speed. Further, it is necessary to use special paper such as coated paper, posing a problem to increase a cost of paper.
In a laser printer, a printing speed is decreased. Moreover, in an electrophotographic system, there is a need to decrease a toner particle size and also attain a high precision in each process. Accordingly, it takes much labor and time to form the high-precision image incapable of reproduction with a laser printer.